Strip-Till No Build-Up Berm Builder Blade For Strip-Till Farm Implement

ABSTRACT

A berm building disc is designed to reduce the amount of soil that builds up on the inner surface of the disc as the disc pushes soil for berm formation. The disc has a disc body having an outer radial edge, and defining an inner concave surface and an outer convex surface. Features are formed on the inner concave surface to reduce the surface area or soil adhesion properties of the inner concave surface.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. Ser. No. 61/522,582filed Aug. 11, 2011.

BACKGROUND OF THE INVENTION

The present invention relates generally to farm implements and, moreparticularly, to tillage implements.

Tillage is an agricultural practice in which the soil (“seedbed”) isprepared for subsequent seeding. The seedbed is prepared by agitatingthe soil using mechanical agitations, such as by digging, stirring, oroverturning. Tillage loosens and aerates the top layer of soil. It alsohelps in the mixing of crop residue, organic matter, and nutrientsthroughout the soil. Tillage is also effective in uprooting anddestroying invasive plantings, e.g., weeds, and also is effective indrying the soil before seeding.

Tillage is often classified into two types: primary and secondary.Primary tillage is typically carried out immediately or shortly afterharvest to mix the crop residue with the soil. Primary tillage is alsodone at greater soil depths and leaves behind a rough and somewhatclumpy surface. Secondary tillage, on the other hand, is typicallycarried out immediately before seeding. It is carried out at shallowersoil depths and yields a smoother surface finish. A couple of theprinciple aims of secondary tillage is to (1) reduce soil compactionthat may have occurred over the winter and (2) dry out the soil.

While tillage can be classified into two different types, in general,there are three different types of tillage systems. Intensive tillagesystems leave less than 15% crop residue cover or less than 500 poundsper acre (560 kg/ha) of small grain residue. These types of tillagesystems are often referred to as conventional tillage systems. Thesesystems involve often multiple operations with implements such as a moldboard, disk, and/or chisel plow. Then a finisher with a harrow, rollingbasket, and cutter can be used to prepare the seed bed. Reduced tillagesystems leave between 15 and 30% residue cover on the soil or 500 to1000 pounds per acre (560 to 1100 kg/ha) of small grain residue duringthe critical erosion period. This may involve the use of a chisel plow,field cultivators, or other implements. The third type of tillagesystems are conservative tillage systems. Conservation tillage systemsare methods of soil tillage which leave a minimum of 30% of crop residueon the soil surface or at least 1,000 lb/ac (1,100 kg/ha) of small grainresidue on the surface during the critical soil erosion period. Thisslows water movement, which reduces the amount of soil erosion.Conservation tillage systems also provide the benefit of reduced fuelconsumption and soil compaction.

One type of conservation tillage system is strip-till, which is intendedto combine the soil drying and warming benefits of conventional tillagewith the soil-protecting advantages of no-till by disturbing only theportion of the soil that is to contain the seed row. Another advantageof strip-till is that many of the strip-till machines can also applychemicals and fertilizer at the same time as tillage.

A conventional strip-till machine or implement consists of a toolbarwith a variety of working tools mounted thereto. These working toolstypically include disc coulters, shanks, sealers (“berm builders”), andconditioning baskets. The disc coulters plough through and cut the cropreside and the shanks cut through the soil. This cutting through thesoil causes the soil to be lifted whereupon the sealers, which aretypically discs often arranged in tandem and angled relative to thedirection of travel, push the raised soil toward an imaginary centerlinebetween the two discs. The pushed soil effectively forms a berm. Forthose machines so-equipped, fertilizer or chemicals are placed in thesoil as the shanks cut through the soil resulting in the berm containinga mixture of soil and chemicals and/or fertilizer. During planting,seeds will be deposited onto the nutrient-rich berms.

The sealers, as noted above, are typically discs (also referred to as“disc blades”). Each disc has an inner or concave surface and an outeror convex surface. For the tandem discs, the raised soil is pushed bythe inner surfaces toward the aforementioned centerline to form a bermgenerally along the centerline. One of the drawbacks of conventionalsealers is that soil tends to collect and build up on the inside orconcave surface of the disc, as illustrated in FIG. 1. Soil tends tobuild up on the concave surface of the disc, starting from the center ofthe disc and continuing to build in an outwardly radial direction to theouter edge of the disc. The soil build-up is much more rapid withwetter/stickier soils or soil conditions. Wetter conditions are moreprevalent in the late fall of the year with fall rains, reduced heatunits to dry the soil, and reduced hours of sunlight. As much of thestrip tillage takes place in the late fall, soil buildup on the sealerscan be quite common.

When soil builds up on the concave side of the sealer disc, the discbecomes more like a roller than a blade and, as a result, the discbegins to ride over the lifted soil rather than direct the soil inwardto build a berm. In strip-till, berm formation and berm height iscritical and thus it is imperative that the berm builder disc besubstantially free of build-up.

SUMMARY OF THE INVENTION

The present invention provides a disc for a sealer that is designed toreduce the amount of soil that builds up on the concave surface of thedisc as the disc pushes the soil for berm formation. In this regard, thepresent invention provides for consistent operation of the disc as thereduced soil build-up on the disc does not substantially impede soilflow. Further, since the amount of soil build-up on the disc is reduced,less time is required to manually clean the disc. The present inventionalso provides a lighter disc and thus reduces the overall weight of thestrip-till implement. A lighter implement reduces the stresses placed onthe implement components and reduces the amount of horsepower requiredto pull/push the implement. Moreover, since the disc of the presentinvention significantly reduces soil accumulation on the concavesurface, the weight from the soil that would otherwise accumulate isalso reduced.

Therefore, in accordance with one aspect of the invention, a disc foruse with a sealer assembly of a strip-till implement is provided. Thedisc has a body with an inner concave surface and an outer convexsurface. The body is configured to rotate about an axis of rotation. Aplurality of holes in radial spaced relation is also formed in the body.In one embodiment of the invention, the holes are centered about theaxis of rotation. The holes reduce the surface area of the disc and thusreduce collection points for soil to build up on during use.

In accordance with another aspect of the invention, a disc for use witha sealer assembly of a strip-till implement is provided. The disc has abody with an inner concave surface and an outer convex surface. The bodyis configured to rotate about an axis of rotation. The inner concavesurface has removed area(s) that reduce the overall surface area of theinner concave surface.

According to another aspect of the invention, a disc for use with asealer assembly of a strip-till implement is provided. The disc has abody with an inner concave surface and an outer convex surface. Aninsert is attached to the inner concave surface and is formed of amaterial to which soil is less likely to adhere to compared to thematerial makeup of the inner concave surface.

Other objects, features, aspects, and advantages of the invention willbecome apparent to those skilled in the art from the following detaileddescription and accompanying drawings. It should be understood, however,that the detailed description and specific examples, while indicatingpreferred embodiments of the present invention, are given by way ofillustration and not of limitation. Many changes and modifications maybe made within the scope of the present invention without departing fromthe spirit thereof, and the invention includes all such modifications.

BRIEF DESCRIPTION OF THE FIGURES

Preferred exemplary embodiments of the invention are illustrated in theaccompanying drawings in which like reference numerals represent likeparts throughout.

In the drawings:

FIG. 1 is an isometric view of a conventional disc blade shown with soilbuild-up;

FIG. 2 is a side elevation view of a conventional working unit of astrip-till implement;

FIG. 3 is a rear elevation view of the working unit shown in FIG. 2;

FIG. 4 is a partial right front perspective view of the working unitshown in FIGS. 2 and 3;

FIG. 5 is a right side elevation view of a disc blade according to oneembodiment of the present invention for use with the working unit ofFIGS. 2-4;

FIG. 6 is a left isometric view of the disc blade of FIG. 5;

FIG. 7 is a plan view of the disc blade shown in FIGS. 5 and 6;

FIG. 8 is an isometric view of a disc blade according to anotherembodiment of the present invention;

FIG. 9 is a left side elevation view of the disc blade of FIG. 8;

FIG. 10 is front elevation view of the disc blade of FIG. 9; and

FIG. 11 is an isometric view of a disc blade according to a furtherembodiment of the present invention.

DETAILED DESCRIPTION

In the following description, right hand and left hand references aredetermined by standing at the rear of the machine and facing in thedirection of forward travel. Also, in the following description, it isto be understood that such terms as “forward,” “left,” “upwardly,” etc.,are words of convenience and are not to be construed as limiting terms.

For purposes of discussion, the term “working unit” will be used hereinto refer to a grouping of apparatus, for example as shown in FIG. 4,that engage the ground in concert to accomplish a certain tasks. In thecase of the instant invention, the task accomplished is the creation ofan improved strip-till seedbed in the form of a raised berm superiorlysuited for planting.

Referring now to FIG. 2, reference numeral 10 generally designates aconventional working unit for strip tillage. In this figure, the variousground-engaging apparatus are shown in a raised or transport position.For comparison, in the working position the tillage point would be inthe ground several inches with the blade and conditioning reelsassemblies generally at ground level. The working unit 10 is attached toan implement frame adapted to be drawn across the ground by a tractor,the forward direction to the right in FIG. 2. The implement frameconventionally includes a long, main tubular frame member, or toolbar,12 that extends transversely to the direction of travel the desiredwidth for the number of working units chosen. Toolbar 12 may compriseone or more sections that, in wider configurations, fold up into a morecompact “package” for transport and storage. The location of the workingunits may be changed when it is desired to alter the spacing betweenadjacent working units, i.e., the working units may be relocatedrelative to one another along the toolbar. Typical implements have 6, 8,12, 16 or more working units mounted to the toolbar at lateral spacingsof 30-36 inches or as determined by planter row spacing. Thisside-by-side arrangement of multiple working units, each operating tocreate a seedbed row or a row of crops, is sometimes referred to as an“indexed” system.

The working unit 10, described with reference to FIGS. 2-4, comprisesfour basic components: (1) a toolbar mounting assembly 14, (2) shankassembly 16, (3) soil gather blade assembly 18, and (4) conditioningrotary reel assembly 20.

Toolbar mounting assembly 14 is comprised of a heavy-gauge steelirregularly shaped coupler 30 with a transverse adjustable opening, ortrack, 32 therethrough that is fitted to the toolbar 12. The coupler 30is sufficiently rigid and affixed to the toolbar such that the remainderof the basic components form a working unit supported by the toolbar.

Shank assembly 16 is comprised of shank 36, a shank bracket 38 includinga pair of side plates 40, 42 affixed to coupler 30 of the toolbarmounting assembly 14, a compression spring 44 affixed at the top end tocoupler 30 at 46 and to shank bracket 38 at the bottom end by bolt 48.The shank bracket 38 is pivotably affixed to toolbar coupler 30 by pin51 such that the shank 36 may pivot a distance limited by the amountthat spring 44 can be compressed. Shank 36 is affixed to shank bracket38 by bolts 52 and 53. Shank 36 is formed of heavy plate stock having athickness of about 1-¼ inches, although this dimension is not criticalto the instant invention.

The shank 36 is characterized herein as being generally curved; however,other configurations could prove satisfactory. In any event, shank 36 isshown to extend forwardly at the base where there is located aground-breaking tillage point 54. The tillage point 54 is commonlyreferred to as a fertilizer knife to accommodate anhydrous ammonia andor other product nutrients. Tillage point 54 is normally run in therange of about 5 to 9 inches deep. The point runs in the compactionlayer, fractures the layer and relocates the soil particles. Shank 36 isadjustable vertically by raising and lowering the toolbar in knownmanner. To protect the shank 36 and other components of the implement 10from severe damage, tillage point 54 is provided with a shear bolt 55that fails upon being subjected to a predetermined force created byimpact between the point 54 and a relatively immovable object such as arock, tree root or the like. Failure of shear bolt 55 allows point 54 topivot rearwardly about bolt 57.

A gathering blade bracket 60 is rigidly fixed at the lower end to bladeyoke 62 and at the upper end to coupler 30 by the pivot bolt 50. Yoke 62has two downwardly extending yoke arms 64, 66 rotatably attached,respectively to blades 68 and 70. These blades may be of any suitabletype; however, concave disc blades are preferred. The blades are angledinwardly, front to back, to gather the soil exploded by the shank 36 andredirect it inwardly toward the conditioning rotary reel. A pair ofadjustable compression springs 72, 74, one on each side of coupler 30,is affixed at the top end to support arm 80 of the conditioning rotaryreel assembly 20 and at the lower end to gathering blade bracket 60.Thus, the blades are biased downwardly. The length of the gathering discbracket 60 is such that the blades 68, 70 are positioned substantiallyadjacent and slightly rearwardly to the shank 36 and the groundbreakingpoint 54. By positioning the pivot point 50 of the gathering bladeassembly 18 forward of the shank 36 the gathering blades are caused tomove mostly upwardly when the depth of tillage point 54 changes (as itconstantly does when crossing a field), thus maintaining the positionalrelationship between the gathering discs and the shank. Morespecifically, as tillage point 54 breaks through the ground, it createsa flow of soil from each side of shank 36, much like snow flowing off ofa V-shaped snowplow. The gathering blades 68, 70 are positioned adjacentand slightly rearwardly of the shank such that they engage this flow ofsoil and redirect it toward the path of the shank for treatment by theconditioning reel. The pivot point of the gathering blade assemblymaintains the positional relationship between the blades and the shankno matter how the depth of the tillage point varies. This positioningprevents soil exploded by the shank point from projecting outside of theblades, and ensures that there is sufficient soil to build a properberm. The blades are adjustable for depth, angle, and distance betweenblades to permit the operator to maximize the containment of soil tobuild a berm. The blades may be dull-edge or sharp-edge, depending uponhow much soil is to be moved. Dull blades tend to ride on top, whilesharp blades tend to cut in.

Conditioning rotary reel assembly 20 is comprised of support arm 80rigidly affixed at the forward end to coupler 30 and extendingrearwardly therefrom in a cantilevered configuration. Support arm 80 isshown as two identical flat bars 82, 84 bolted to either side of coupler30 and pivotably connected to reel arm 90 at pivot bolt 86. Anadjustable compression spring 88 extends between support arm 80 and astop plate 91 rigidly affixed to reel arm 90 such that the reel arm 90may pivot about pivot bolt 86 to adjustably absorb movement of therotary reel due to uneven areas of the ground or impacts with obstacleson the field. Most importantly, however, compression spring 88 adjuststhe down pressure for controlling clod sizing.

A coulter (not shown) is affixed forwardly of the shank 36 to eithertoolbar 12 or another toolbar or structural member of the implementframe in know fashion. Numerous examples exist in the prior art, butexamples of such a coulter are shown in U.S. Pat. Nos. 5,797,460 and6,102,132 and would prove satisfactory in the instant application. Acoulter thus located results in cutting and sizing residue. The depth ofcut of the coulter would be adjustable upwardly or downwardly within aclamp, and adjusted to accommodate or compensate for various settings ofthe tillage depth or soil conditions. General practice would be to use acoulter with a diameter of approximately 20 to 24-inches. The coultersmay be either wavy or smooth depending upon desired results and workingconditions; smooth being preferred. For in-row root cutting and lesssurface disruption, a smooth coulter would be selected. In any event,the primary purpose of the coulter is to cut the residue and cut agroove into the soil to enhance soil flow at the tillage point andequalize soil flow off of each side of the tillage point.

A conditioning rotary reel 100 conditions the strip of soil to shape andpre-settle the berm, to give ideal seed-to-soil contact and uniform bermsize, thus promoting early, fast and uniform emergence. The rotary reel100 is mounted to a yoke 102 on bearings to provide smooth and freerelative rotation of the reel. The yoke 102 is affixed to reel arm 90pivotally mounted to support arm 80. The reel assembly 20 is shown toinclude an adjustable spring 88, which would be adjusted as needed tomaintain firm engagement between the conditioning rotary reel and theground sufficient to break up clods. Generally, the larger the clods inthe field, the greater the down pressure required. If down-pressure isrequired to augment the weight of the reel itself, any appropriatemechanism or approach can be used, such as, for example, spring packs orweights.

As the shank cuts through the soil, it explodes the soil outwardly whereit contacts the gathering blades and is redirected inwardly toward thepath of the shank. If the surface of the ground is level and the densityis generally uniform, the soil exploded is uniform on the two sides ofthe shank, and is uniform when turned inwardly by the blades; however,as is more often the case, the ground is not level and the soil is notof uniform density, the exploded soil is not of equal volume on the twosides of the shank and, thus, the soil turned by the blades is notuniform and of equal volume. This lack of uniformity of volume anddensity causes the blade being hit by the higher volume to throw moresoil inwardly, and to actually throw some soil all the way across theshank path and onto an untilled area of the ground. This situationpresents a problem in that it does not provide sufficient volume of soilto build the desired berm. To avoid this, the conditioning rotary reelmust be located close enough to the blades to catch the extra volume ofsoil and encompass it into the berm. Generally, this requires theconditioning rotary reel to be located in the range of 6 to 10 inchesfrom the back of the blades.

Still referring to FIGS. 2-4, rotary reel 100 can be seen to include apair of opposing end plates 104, 106. The end plates may be notched orscallop-shaped to reduce plugging in wet conditions and avoid creationof a groove beside the berm. The end plates are partially maintained intransverse alignment by a support shaft 108 that extends throughcentrally located apertures in each of the end plates. Support shaft 108is affixed, as by welding, to each of the end plates and extends beyondeach to fit into bearings on yoke 102. A plurality of elongateground-engaging bars 110, made from flat stock, extends from end-to-endacross the end plates and are affixed thereto. The end plates haveslots, as at 112 in FIG. 2, into which the bars fit. After welding, thisarrangement provides superior support and strength for the connection.While it is preferred that the bars be fabricated from flat stock,satisfactory results can be obtained by the use of other structures,such as, for example, bar stock or lengths of tubular material. Itshould be obvious to one of ordinary skill in the art that other methodsand arrangements for affixing the bars to the end plates would provide asatisfactory structure.

FIGS. 2-4, as described above, show a working tool having conventionalberm forming blades, which are designated in the figures by referencenumerals 68 and 70. As shown in those figures, each blade 68, 70 has asolid inner concave surface, which as discussed above, can result insoil capture and build-up on the concave surface, such as that shown inFIG. 1. Accordingly, the present invention provides a concave blade 114having a relatively thin body 116 with an outer radial edge 118, anddefining an inner concave surface 120 and an outer convex surface 122,as shown in FIGS. 5-7. As best shown in FIG. 7, a central opening 124 isformed in the center of the body 116 and is used for mounting theconcave blade 114 to an axle assembly 126, which is shown in FIG. 5 andgenerally known in the art.

In a preferred embodiment, portions of the body 116 are removed toreduce the surface area of the inner concave surface 120. In theillustrated embodiment, this reduction in surface area is achieved bycutting or otherwise forming holes 128 in a spaced radial arrangementaround the central opening 124. The holes 128 are preferably located asnear to the center, i.e., central opening 124, of the blade body aspossible without interfering with the mounting of the blade to the yokearm. By reducing the surface area at the center of the inner concavesurface 120, there is a reduced amount of blade material for soil toadhere to during tillage. By keeping the center area of the innerconcave surface 120 substantially free of soil build-up (or at leastsignificantly reducing the amount of soil build-up), soil build-up isalso reduced at the outer radial portion of the blade, which preventsthe blade from operating like a roller instead of a blade.

In one embodiment, the radial distance from the center of the centralopening 124 to the center of each hole 128 is approximately 4.5 inchesand the diameter of each hole 128 is approximately 2.375 inches. In oneembodiment, six holes 128 are formed equiradially around the centralopening 124, i.e., at 60 degree intervals from the origin of the centralopening; however, other arrangements are contemplated.

FIGS. 8-10 show a berm forming concave blade 130 according to anotherembodiment of the invention. Like the embodiment described above withrespect to FIGS. 5-7, concave blade 130 has a relative thin body 132having an inner concave surface 134 and an outer convex surface 136.Rather than a continuous curved outer edge, concave blade 130 has ascalloped outer radial edge 138 in which a series of notches 140 or“scallops” are cut, or otherwise formed, in the radial edge 138. Concaveblade 130 also has a series of surface area-reducing holes 142 centeredabout a mounting opening (not shown) and mounting axle 144. In theillustrated embodiment, the outer radial edge 138 has eight notches 140.

The heretofore described holes reduce the surface area of the innerconcave surface and thus reduce collection points for soil. This reducesthe amount of soil that would otherwise collect on the inner concavesurface of the blades, which is apparent from comparing FIG. 1 withFIGS. 6 and 8. It is understood that the surface area-reducing holesdescribed above represent but one manner in which the surface area ofthe inner concave surfaces could be reduced. For example, slits/slotscould be cut into the blade. Additionally, while circular holes areshown and have been described, it is understood that the surface-areareducing features could have other geometric or complex shapes.Additionally, it is contemplated that a single disc could have more thanone type or one shape of surface-area reducing feature. Moreover, it iscontemplated that the surface-area reducing features could be varyingsizes.

Turning now to FIG. 11, a disc blade 146 according to another embodimentof the invention has a circular body 148 with an outer radial edge 150.The circular body 148 has an inner concave surface 152 and an outerconvex surface 154. To reduce soil collection on the inner concavesurface 152, an insert 156 is attached to the inner concave surface 152.In one embodiment, fasteners 158, e.g., screws, are used to secure theinsert 156 to the circular body 148. Alternately, suitable bondingagents could be used. The insert is mounted at the center of the innerconcave surface 152, and has a generally circular body 160 formed ofplastic, polyurethane, or other corrosion resistant material. It ishighly preferred that the outer surface of the insert 156 is formed of amaterial having low soil adhesion properties. Having the outer surfaceof the insert “slicker” than the inner concave surface resists soilbuildup on the insert and thus the inner concave surface. The insert ispreferably made of lightweight material so that the weight of the bladeis not substantially increased.

The present invention has been described with respect to a disc for usewith a strip-tillage implement but it is understood that the inventioncould be used with other types of farming implements, such as discsealers, disk rippers, and levelers to name a few. Additionally, whilethe invention has been described with respect to a concave disc, it isunderstood that the invention is not so limited. For example, theinvention could also be used with flat discs.

Many changes and modifications could be made to the invention withoutdeparting from the spirit thereof. The scope of these changes willbecome apparent from the appended claims.

1. A disc blade for a farming implement, comprising: a body having aninner surface, an outer surface, and a radial outer edge; and aplurality of holes extending through the body between the inner surfaceand the outer surface.
 2. The disc blade of claim 1 further comprising acentral opening formed in the body and adapted for receiving a mount forcoupling the disc blade to the strip-till implement, and wherein theplurality of holes are arranged in a radial spaced arrangement aroundthe central opening.
 3. The disc blade of claim 2 wherein the pluralityof holes includes six holes spaced on nine inch centers around thecentral opening.
 4. The disc blade of claim 1 wherein each of theplurality of holes has a diameter between 2.0-2.5 inches.
 5. The discblade of claim 1 further comprising a plurality of notches formed in theradial outer edge.
 6. The disc blade of claim 1 wherein the innersurface has an inner radial portion and an outer radial portion, andwherein the plurality of holes is formed in the inner radial portion. 7.The disc blade of claim 1 wherein the inner surface is a concave surfaceand the outer surface is a convex surface.
 8. A disc blade for a farmingimplement, comprising: a body having an inner surface, an outer surface,and a radial outer edge; and means for reducing the surface area of theinner surface.
 9. The disc blade of claim 8 wherein the reducing meansincludes a plurality of holes formed in the inner surface.
 10. The discblade of claim 9 further comprising a central opening formed in the bodyand adapted for receiving a mount for coupling the disc blade to thefarming implement, and wherein the plurality of holes are arranged in aradial spaced arrangement around the central opening.
 11. The disc bladeof claim 10 wherein the plurality of holes includes six holes spaced onnine inch centers around the central opening.
 12. The disc blade ofclaim 9 wherein each of the plurality of holes has a diameter between2.0-2.5 inches.
 13. The disc blade of claim 9 wherein the inner surfaceis a concave surface and has an inner radial portion and an outer radialportion, and wherein the plurality of holes is formed in the innerradial portion.
 14. The disc blade of claim 8 further comprising aplurality of notches formed in the radial outer edge.
 15. The disc bladeof claim 8 wherein the body has a thickness between one-tenth of an inchand four-tenths of an inch.
 16. A working unit for a farming implementhaving a tool bar, comprising: a coupler for mounting to the tool bar; abracket attached to the coupler; and a disc rotatably attached to thebracket, the disc including a body having an inner surface, an outersurface, and a radial outer edge, and a plurality of holes extendingthrough the body between the inner surface and the outer surface. 17.The working unit of claim 16 wherein the disc further includes a centralopening formed in the body and adapted for receiving a mount forcoupling the disc blade to the bracket, and wherein the plurality ofholes are arranged in a radial spaced arrangement around the centralopening.
 18. The working unit of claim 17 wherein the plurality of holesincludes six holes spaced on nine inch centers around the centralopening.
 19. The working unit of claim 16 wherein the inner surface isconcave and the outer surface is convex.